1. Field of the Invention
This invention relates to a semiconductor device provided with a package of the ball grid array (BGA) type and comprising semiconductor chips connected by a flip-chip process to a resin substrate with a number of solder ball terminals.
2. Description of the Prior Art
Packages for packaging semiconductor chips have recently been provided with an increasing number of base pins (lead wires). As one of such packages, there has been provided a package of the pin grid array (PGA) type in which pins are disposed into the shape of a grid on the overall backside of the package. In the PGA type package, a ceramic substrate is provided with a number of base pins embedded therein and includes a mounting portion on which a number of electrode pads are provided. The pins are connected via conductor patterns to the electrode pads respectively. A semiconductor chip is mounted onto the mounting portion of the substrate by a flip-chip process.
In mounting the semiconductor chip by the flip-chip process, it is important to be able to dissipate or radiate heat produced by the mounted chip not only from a mounted side thereof but also from its backside opposite the mounted side. For this purpose, a container-shaped cover plate is made of a metal such as aluminum, for example. The cover plate is bonded to an upper side of the semiconductor chip and further at a periphery thereof to the substrate. Heat produced by the semiconductor chip is transferred from the backside or upper side thereof via an adhesive agent to the cover plate, from which the heat is radiated outward. The cover plate serves to mechanically and electrically protect the semiconductor chip as well as to radiate heat therefrom.
Various packages of the ball grid array (BGA) type have recently been developed instead of the above-described PGA type packages. The BGA type packages have recently drawn attention as those of the surface mounting type. In the BGA type packages, a number of ball-shaped solder terminals (solder balls) are provided on connecting terminal conductors disposed into the shape of a grid, instead of the pins. The solder balls are disposed opposite the connecting terminal conductors respectively when the BGA type package is mounted on the printed board. The solder balls are soldered in the block by reflow processing to be electrically connected to the respective conductors.
The soldered terminals are sometimes subjected to mechanical stress in the BGA type package since the BGA type package is mounted on the substrate without using terminals so as to be closely adherent to the substrate. The package needs to be rendered large in size with increase in the size of the semiconductor chip. However, when a large-sized ceramic substrate is mounted on the printed board, mechanical stress due to changes in the temperatures of the substrate and the board is increased to such an extent that the stress cannot be ignored. The solder terminals located particularly at ends of the package are subjected to a larger stress than those located at the center of the package. This sometimes results in occurrence of cracks in connecting portions of the solder or in disconnection or circuit opening.
The above-described drawbacks result from the difference in coefficients of linear expansion between the printed board and the ceramic. To overcome the drawbacks, the prior art has proposed use of a resin substrate as the substrate of the package on which the semiconductor chip is mounted. By the use of the resin substrate, the mechanical stress to which the connecting portions of the solder is subjected can be eased or reduced. Furthermore, the size of the substrate of the package can be increased, so that the limitation in the size of the semiconductor chip can be reduced.
However, stress between the resin substrate and the radiating cover plate sometimes warps the substrate when the cover plate is attached to the resin substrate. Particularly when the cover plate is made of a metal such as aluminum, the stress deforms the resin substrate while the bonded cover plate is being cooled. This stress adversely affects the semiconductor chip, resulting in a new drawback.
It is important to maintain a desirable heat conductivity between the cover plate and the semiconductor chip so that heat generated by the semiconductor chip is efficiently radiated through the cover plate. Accordingly, the heat conductivity of the cover plate needs to be taken into consideration. In view of this, a process as shown in FIGS. 14 and 15 is employed, for example. Prior to a step of attaching a cover plate 1 to a resin substrate 2, a predetermined amount of paste 3 with high heat conductivity is applied to an inside of the cover plate 1 by a dispenser 4 (see FIG. 14). In this state, the cover plate 1 is bonded by an adhesive agent 7 to the resin substrate 2 on which the semiconductor chip 5 has been mounted (see FIG. 15). The inside portion of the cover plate 1 to which the paste 3 is applied is maintained in contact with the semiconductor chip 5.
The step of applying the paste 3 to the cover plate 1 is an independent step in an ordinary assembly line since only the cover plate is processed in the step. Furthermore, this step requires new equipment depending upon conditions such as the shape and size of the cover plate 1, resulting in an increase in the cost.
Furthermore, when having a low viscosity, the paste 3 overflows a predetermined applying area on the inside of the cover plate 1, so that it becomes difficult to suitably apply a necessary amount of paste 3 to the cover plate.
Therefore, an object of the present invention is to provide a semiconductor device provided with the BGA type package including a cover plate wherein the above-described drawbacks due to warp of the substrate can be prevented.
Another object of the invention is to provide a semiconductor device provided with the BGA type package wherein a high-temperature conductive paste can, even if it has a low viscosity, be applied to the cover plate without overflowing and can readily be applied by the conventional equipment with no specific equipment being required.
The present invention provides a semiconductor device comprising a substrate made of a resin and having one side on which a number of solder ball terminals are provided and the other side on which a chip mounting portion electrically connected to the solder ball terminals is provided, and a cover plate made of a metal and attached to a semiconductor chip so as to cover and come into contact with the same under a condition where the semiconductor chip is connected to the resin substrate by a flip-chip process, the cover plate including a base brought into contact with the semiconductor chip and a peripheral portion provided with a plurality of bonding portions where the cover plate is bonded to the substrate, the bonding portions being discontinuous to each other.
According to the above-described construction, the cover plate includes a plurality of the peripheral discontinuous bonding portions where the cover plate is bonded to the substrate to which the semiconductor chip is connected. Stress due to temperature changes occurs between the substrate and the cover plate when the cover plate is bonded to the substrate. However, the above-described construction can reduce the stress as compared with the conventional construction in which the cover plate has a continuous bonding portion. Consequently, the warp of the substrate due to the bonding of the cover plate thereto can be reduced even when the substrate is made of a resin. Furthermore, even when a large-sized substrate is used, the structure of the semiconductor device can be rendered practically durable.
In one preferred form, the bonding portions are arranged into an axial symmetry. Consequently, since the stress is evenly shared by the bonding portions, the stress can efficiently be dispersed.
In another preferred form, the cover plate includes at least one connecting portion connecting between the base and the bonding portions and is formed by bending a metal plate so that the base, and the bonding portions are connected by the connecting portion such that the base, bonding portions and connecting portion are formed integrally with one another. Consequently, the number of parts can be reduced since the cover plate is obtained from a single metal plate. Furthermore, the connecting portion is preferably divided into a plurality of portions connecting the respective connecting portions to the base. Consequently, the stress can be reduced as compared with the case where the cover plate with a continuous bonding portion is formed of a single metal plate by drawing, whereupon the warp of the substrate can be reduced.
In further another preferred form, each divided portion of the connecting portion has a width smaller than a length of the corresponding bonding portion. In this construction, the stress produced in the base is absorbed by the connecting portions, so that the stress produced between the cover plate and the resin substrate can be eased and accordingly, the warp of the substrate can be reduced.
In the above-described construction, an amount of heat transferred through the connecting portions is decreased since the connecting portions include reduced width portions respectively. Accordingly, the construction should be employed when the reduction in the stress has priority over the heat radiation.
In further another preferred form, the connecting portion is formed continuously so as to surround the base. The base and the connecting portion serving to cover the semiconductor chip are formed into the shape of a container. Consequently, this construction is effective in the protection of the semiconductor chip in addition to provision of the reduction in the stress.
In further another preferred form, the cover plate is made of a metal having a linear expansion coefficient equal to or larger than one of the resin substrate and equal to or smaller than 20 ppm/xc2x0 C. When the cover plate bonded to the resin substrate is made of a metal with the above-described range of the linear expansion coefficient, the warp of the substrate can effectively reduced.
The cover plate is preferably made of copper (Cu) or stainless steel. Either metal meets the above-mentioned range of the linear expansion coefficient (Cu: 17.6 ppm/xc2x0 C.; and SUS: 17.3 ppm/xc2x0 C.). Since each metal has a good heat conductivity, the heat radiating effect can be improved.
Each bonding portion preferably has such a minimum width that a predetermined bonding strength is ensured between the resin substrate and each bonding portion. When the reduction in the warp is taken into consideration, the width of each bonding portion should be rendered smaller. However, a predetermined bonding strength needs to be ensured in each connecting portion. Accordingly, it is effective to render the width of each connecting portion smaller so far as the predetermined bonding strength is ensured.
Each bonding portion of the cover plate preferably has a width ranging between 2 mm and 4 mm. Consequently, the warp of the substrate can desirably be reduced even when the resin substrate 40 mm square is used.
The invention also provides a semiconductor device comprising a substrate made of a resin and having one side on which a number of solder ball terminals are provided and the other side on which a chip mounting portion electrically connected to the solder ball terminals is provided, and a cover plate made of a metal and attached to a semiconductor chip so as to cover and come into contact with the same, the semiconductor chip being connected to the resin substrate by a flip-chip process, the cover plate including a base brought into contact with the semiconductor chip and a peripheral portion provided with a plurality of bonding portions where the cover plate is bonded to the substrate, the bonding portions being discontinuous to each other, the cover plate including a barrier preventing a paste for increasing heat conductivity from overflowing a portion thereof corresponding to a location of the semiconductor chip the paste being provided between the semiconductor chip and the cover plate when the chip is mounted on the substrate.
According to the above-described construction, even when the paste applied to the cover plate for improvement in the heat conductivity has a low viscosity, the barrier accommodates the predetermined amount of paste therein, retaining the paste while the cover plate is bonded to the substrate. Consequently, a necessary amount of paste can be used effectively and steadily, and the paste can be prevented from flowing outside the barrier. The barrier is preferably formed integrally with the cover plate. Alternatively, an adhesive sheet is preferably applied to substantially the overall face except a portion thereof brought into contact with the semiconductor chip, whereby the barrier is formed.
The invention further provides a semiconductor device comprising a substrate made of a resin and having one side on which a number of solder ball terminals are provided and the other side on which a chip mounting portion electrically connected to the solder ball terminals is provided, and a cover plate made of a metal and attached to a semiconductor chip so as to cover and come into contact with the same. The semiconductor chip is connected to the resin substrate by a flip-chip process. The cover plate includes a base brought into contact with the semiconductor chip and a peripheral portion provided with a plurality of bonding portions where the cover plate is bonded to the substrate. The bonding portions are discontinuous to each other. The cover plate has an injection opening through which a paste for improvement in heat conductivity is supplied into an area of a space defined between the cover plate and the resin substrate when the cover plate is attached to the resin substrate. The area corresponds to a location of the semiconductor chip.
According to the above-described construction, the paste for improvement in the heat conductivity can be supplied through the injection opening after the cover plate has been bonded to the resin substrate. This eliminates equipment for applying the paste to the cover plate previously. Consequently, the semiconductor device can be fabricated with use of no special assembling equipment.